Fiber optics has drawn increasing attention from researchers as a medium for communications, and its technology has advanced to such a level that commercial optical fibers are now fabricated. However, studies are still being made on the method of reducing the transmission loss of the fiber.
A conventional apparatus for producing an optical fiber preform is illustrated schematically in FIG. 1 wherein a muffle furnace 1, i.e., a protective reaction vessel (usually having an inner volume of about 2 l to about 200 l) with an oxyhydrogen burner 2 disposed in the bottom thereof. The burner typically has a plurality of nozzles through which a glass forming material and a dopant for modifying the refractive index of the glass are issued upward to provide a predetermined distribution of gases in the muffle interior. The glass forming material is hydrolyzed (oxidized) with the flame from the burner and soot 4 deposits, initially on the tip of a supporting rod 3 facing the burner and then on the deposited face of the preform, as the rod is pulled up while it is rotated. However, as a result of the flame hydrolysis, reaction gases such as water vapor, hydrogen, chloride gas, and chlorine gas are formed in the muffle furnace, and these gases have adverse effects on the refractive index distribution, and other properties of the preform. To prevent this, the muffle furnace 1 is provided with an opening 5 through which air is supplied and an exhaust port 6 through which the undesired gases are discharged. The exhaust port 6 is connected to an exhaust pipe 7 which is connected to an exhaust gas treating means 8 (usually having a cross-sectional area of about 2 cm.sup.2 to about 80 cm.sup.2) which is further connected to a fan 9 that exhausts the exhaust gas from the apparatus. The exhaust gas treating means 8 is generally a washing tower where the exhaust gas is contacted with water or an aqueous sodium hydroxide solution whereby the hydrogen chloride gas produced by the reaction between silicon chloride and hydrogen-oxygen was neutralized with the aqueous sodium hydroxide solution to prevent environmental pollution. In this arrangement, the gas is supplied through the opening 5, and after the reaction gases are treated by the means 8, they are then discharged into the external atmosphere. However, the reaction gases are not the only product that is brought into the means 8 through the pipe 7; the glass soot and hydrogen chloride gas are also carried into the means 8 through the pipe 7, and as a result, the soot builds up and the gas condenses on the inner wall of the pipe 7, to increase the resistance to the passage of the waste gas. If the waste gas is continuously discharged from the treating means 8 by the action of the fan 9, the increasing resistance of the inner wall of the pipe 7 causes a variation in the amount of the waste gas being discharged through the exhaust port 6, and hence, the amount of the gas being supplied through the opening 5 varies. As a consequence of this, the shape of the flame from the burner 2 is changed, or the position of the flame relative to the glass soot is changed. Furthermore, the temperature in the muffle furnace, and particularly the surface temperature of the soot 4, is varied, to eventually cause a change in the distribution of fine glass particles in the soot and/or in the distribution of refractive index of the product fiber. For these reasons, it has been difficult to produce a high-quality preform by the conventional process.